Semiconductor device

ABSTRACT

A semiconductor device includes: a semiconductor element that includes an electrode layer on a surface of the semiconductor element; a low-strength layer that is provided on a surface of the electrode layer; a bonding layer that is provided on a surface of the low-strength layer; and a conductive plate that is provided on a surface of the bonding layer. Strength of the bonding layer is higher than strength of the electrode layer, and strength of the low-strength layer is lower than the strength of the electrode layer.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2012-261126 filed onNov. 29, 2012 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device.

2. Description of Related Art

For example, Japanese Patent Application Publication No. 2011-129619 (JP2011-129619 A) discloses a semiconductor device in which a conductiveplate is fixed on an electrode layer provided on a surface of asemiconductor element with a solder layer therebetween.

When the conductive plate is fixed on a semiconductor element driven athigh temperatures with the solder layer therebetween, the solder layercan be a high-melting point solder material. However, the high-meltingpoint solder material has higher strength than the electrode layer ingeneral. Therefore, when thermal stress is repeatedly exerted on theelectrode layer and the solder layer due to heat generation and heatdissipation of the semiconductor element, the electrode layer can bedamaged prior to the solder layer.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor device that can inhibitthe electrode layer from being damaged.

A first aspect of the present invention relates to the semiconductordevice. The semiconductor device includes: a semiconductor element thatincludes an electrode layer on a surface of the semiconductor element; alow-strength layer that is provided on a surface of the electrode layer;a bonding layer that is provided on a surface of the low-strength layer;and a conductive plate that is provided on a surface of the bondinglayer. Strength of the bonding layer is higher than strength of theelectrode layer, and strength of the low-strength layer is lower thanthe strength of the electrode layer.

In the aforementioned aspect, the strength of the bonding layer ishigher than that of the electrode layer, and the strength of thelow-strength layer is lower than that of the electrode layer. Therefore,when the thermal stress is repeatedly exerted on the electrode layer andthe bonding layer due to the heat generation and the heat dissipation ofthe semiconductor element, the low-strength layer is easily damagedprior to the electrode layer. Thus, the damage of the electrode layercan be inhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a cross-sectional view that shows a principal part of asemiconductor device according to an example of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Major technical elements of examples according to the present inventionwill be described below. It should be noted that the technical elementsdescribed below are independent of one another and exert technicalusefulness independently or in combination with various elements.Therefore, the present invention is not limited to the combination thatis described in the claims of the application as originally filed.

(Technical Element)

The electrode layer may be made of AlSi, and the low-strength layer maybe made of Al.

Incidentally, the ten “strength” herein means the strength against thestress acting upon each layer. For example, the strength may be definedby the magnitude of yield strength (yield stress). It can be said thatthe strength is higher as the yield strength is higher. In addition,when a material in which the yield strength is not easily defined (amaterial in which a yield point is not clearly defined) is used, it isknown that 0.2% proof stress may be used as a substitute for the yieldstrength. In this specification, the 0.2% proof stress may be describedas an example of the “strength.” The magnitude of the 0.2% proof stresscan be measured through a testing method (tensile test) designated byJapanese Industrial Standards (JIS), for example.

Furthermore, the “strength” can be defined by the magnitude of fatiguestrength, for example. It can be said that the strength is higher as thefatigue strength is higher. In this specification, the fatigue strengthmay be described as an example of the “strength.” The magnitude of thefatigue strength can be measured through a testing method (fatigue test)designated by JIS, for example.

A semiconductor device 10 shown in FIG. 1 includes a semiconductorelement 20, a low-strength layer 40, a bonding layer 50, and aconductive plate 60. In the semiconductor device 10 of examples in thepresent invention, conductive plates 60 are fixed on the front and theback sides of the semiconductor element 20 with the low-strength layers40 and bonding layers 50 therebetween. In the following examples, anupper surface and a lower surface in the drawing may respectively bereferred to as a “front side” and a “back side” with respect to thesemiconductor element 20, the low-strength layers 40, the bonding layers50, and the conductive plates 60.

The semiconductor element 20 includes a semiconductor substrate 22 andelectrode layers 30 that are provided on the front and the back sides ofthe semiconductor substrate 22. The semiconductor substrate 22 is madeof silicon (Si), for example. The electrode layer 30 may be made ofAlSi. In the examples of the present invention, the semiconductorelement 20 is a vertical insulated-gate bipolar transistor (IGBT). Inother examples, the semiconductor element 20 may be other powersemiconductor elements (such as a rectifier diode and a MOSFET). In theexamples of the present invention, the semiconductor element 20 isdriven at high temperatures.

The low-strength layers 40 are provided on the front and the back sidesof the semiconductor element 20. In other words, the low-strength layers40 are provided on the front side of the electrode layer 30 that ispositioned on the upper side in the drawing and the back side of theelectrode layer 30 that is positioned on the lower side in the drawing.In the examples of the present invention, the low-strength layer 40 ismade of Al. The low-strength layers 40 are formed by sputtering Al ontothe front and the back sides of the semiconductor element 20. Thestrength of the low-strength layer 40 made of Al is lower than that ofthe electrode layer 30 made of AlSi. Hereinafter, the examples of thepresent invention are exemplified with a case where the 0.2% proofstress is used as a criterion of the “strength”. That is, in theexamples of the present invention, the 0.2% proof stress of thelow-strength layer 40 is lower than that of the electrode layer 30.

The bonding layers 50 are provided on the front side of the low-strengthlayer 40 that is positioned on the upper side in the drawing and theback side of the low-strength layer 40 that is positioned on the lowerside in the drawing. The bonding layer 50 is made of Sn—Sb based solder.Therefore, the 0.2% proof stress of the bonding layer 50 is higher thanthat of the electrode layer 30 made of AlSi. In other examples, thebonding layer 50 can be any layers as long as the layer has higher 0.2%proof stress than the electrode layer 30 and a relatively high meltingpoint, including a Zn—Al based solder layer, an Ni nanoparticle sinteredcompact layer, an Ag nanoparticle sintered compact layer, and a TLPbonding (liquid phase diffusion bonding) layer in which SnCu or SuNi isused as an insert material. In the examples of the present invention,the melting point of the bonding layer 50 is higher than the temperatureduring the driving of the semiconductor element 20.

The conductive plates 60 are provided on the front side of the bondinglayer 50 that is positioned on the upper side in the drawing and theback side of the bonding layer 50 that is positioned on the lower sidein the drawing. The conductive plate 60 is a lead frame that is made ofCu. The conductive plate 60 functions as a heat sink that dissipatesheat generated by the semiconductor element 20. The conductive plate 60is fixed on the low-strength layer 40 with the bonding layer 50therebetween.

As described above, in the examples of the present invention, the 0.2%proof stress of the bonding layer 50 is higher than that of theelectrode layer 30, and the 0.2% proof stress of the low-strength layer40 is lower than that of the electrode layer 30. Therefore, when thethermal stress is repeatedly exerted on the electrode layer 30 and thebonding layer 50 due to the heat generation and the heat dissipation ofthe semiconductor element 20, the low-strength layer 40 is damaged priorto the electrode layer 30. Thus, the damage of the electrode layer 30can be inhibited.

EXPERIMENTAL EXAMPLE

Hereinafter, experiments in which the present inventor has performed inorder to verify the effects of the semiconductor device 10 of theexamples are described. In these examples, as shown in the followingExamples 1 through 6, semiconductor devices 10 that included variousbonding layers 50 were prepared. On each semiconductor device 10,temperature cycling from 200° C. to −40° C. was repeated for 3,000cycles, and damaged portions were verified. For comparison, as shown inthe following Comparative Examples 1 through 6, semiconductor devicesthat included various bonding layers 50 but excluded the low-strengthlayers 40 were prepared. The same experiments were performed to therespective semiconductor devices.

Examples 1 through 6 are the semiconductor device 10 shown in FIG. 1.The bonding layers 50 of the Examples 1 through 6 were prepared asfollows. In addition, the electrode layer 30 was made of AlSi, and thelow-strength layer 40 was made of Al. Thus, all the 0.2% proof stressesof the bonding layers 50 of the Examples 1 through 6 were higher thanthose of the electrode layers 30 and lower than those of thelow-strength layers 40.

Example 1

Layer of Sn 13-6 SbCu (Sn—Sb based solder)

Example 2

Layer of Zn 6-4 Al (Zn—Al based solder)

Example 3

Layer of Ni nanoparticle sintered compact

Example 4

Layer of Ag nanoparticle sintered compact

Example 5

TLP bonding layer (SnCu is used as the insert material.)

Example 6

TLP bonding layer (SuNi is used as the insert material.)

Comparative Examples 1 through 6 are semiconductor devices in which thelow-strength layers 40 have been excluded from the semiconductor device10 shown in FIG. 1. That is, in the Comparative Examples 1 through 6,the conductive plate 60 was directly fixed on the electrode layer 30made of AlSi with the bonding layer 50 therebetween. The bonding layers50 of the Comparative Examples 1 through 6 were prepared in the samemanners as the bonding layers 50 of the Examples 1 through 6.

Comparative Example 1

Layer of Sn 13-6 SbCu (Sn—Sb based solder)

Comparative Example 2

Layer of Zn 6-4 Al (Zn—Al based solder)

Comparative Example 3

Layer of Ni nanoparticle sintered compact

Comparative Example 4

Layer of Ag nanoparticle sintered compact

Comparative Example 5

TLP bonding layer (SnCu is used as the insert material.)

Comparative Example 6

TLP bonding layer (SuNi is used as the insert material.)

On each of the Examples 1 through 6 and the Comparative Examples 1through 6, temperature cycling from 200° C. to −40° C. was repeated for3,000 cycles. As a result, the low-strength layers 40 made of Al weredamaged in the Examples 1 through 6, and the electrode layers 30 made ofAlSi were damaged in the Comparative Examples 1 through 6.

The experimental results described above reveal the following facts. Inthe semiconductor device 10 of the examples of the present invention,the low-strength layer 40 is provided between the electrode layer 30 andthe bonding layer 50, and the 0.2% proof stress of the low-strengthlayer 40 is lower than that of the electrode layer 30. Therefore, evenwhen the thermal stress is repeatedly exerted on the electrode layer 30and the bonding layer 50, the low-strength layer 40 is damaged prior tothe electrode layer 30. Thus, the damage of the electrode layer 30 canbe inhibited.

The semiconductor device 10 of the examples in the present invention hasbeen described so far. Now, a correspondence relationship between theexamples and claims will be described. Both of the front and the backsides of the semiconductor element 20 in the examples are examples ofthe “surfaces (of the semiconductor element)”. Both of the front side ofthe electrode layer 30 that is positioned on the upper side in thedrawing and the back side of the electrode layer 30 that is positionedon the lower side in the drawing are examples of the “surfaces of theelectrode layer”. Both of the front side of the low-strength layer 40that is positioned on the upper side in the drawing and the back side ofthe low-strength layer 40 that is positioned on the lower side in thedrawing are examples of the “surfaces of the low-strength layer”. Bothof the front side of the bonding layer 50 that is positioned on theupper side in the drawing and the back side of the bonding layer 50 thatis positioned on the lower side in the drawing are examples of the“surfaces of the bonding layer”.

The technique of the present invention has been disclosed and describedin detail in this specification with reference to examples thereof.However, it is to be understood that those examples are merelyillustrative and claims of the present invention are not limited tothose examples. Techniques that are disclosed in the claims of thepresent invention are intended to cover various modifications andchanges of the example embodiments that are described above. Forexample, the following modifications may be used.

Modification 1

In the examples described above, the 0.2% proof stress is used as thecriterion that represents the strength of the electrode layer 30, thelow-strength layer 40, and the bonding layer 50. However, the presentinvention is not limited to this, and any other criteria that representthe strength of the electrode layer 30, the low-strength layer 40, andthe bonding layer 50 can be used as long as the criteria represent thestrength against the stress acting upon each layer (breaking strength).For example, the fatigue strength can be used as the criterion ofstrength. In such a case, the fatigue strength of the bonding layer 50may be higher than that of the electrode layer 30, and the fatiguestrength of the low-strength layer 40 may be lower than that of theelectrode layer 30.

Modification 2

The circumference of the semiconductor device 10 may be sealed withresins. As sealing resin materials in this case, resins such as epoxyresins, polyimide resins, and polyamide resins can be used.

Modification 3

In the examples described above, the low-strength layers 40, the bondinglayers 50, and the conductive plates 60 are provided on both of thefront and the back sides of the semiconductor element 20. However, thepresent invention is not limited to this, and the low-strength layer 40,the bonding layer 50, and the conductive plate 60 may be provided ononly one side of the semiconductor element 20.

In addition, the technical elements that are disclosed in thespecification and the drawings exhibit technical usefulness alone or invarious combinations and configurations. The techniques that areillustrated in this specification and the drawings achieve a pluralityof objects simultaneously, and the achievement of one object thereofitself has technical usefulness.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor element that includes an electrode layer on a surface ofthe semiconductor element; a low-strength layer that is provided on asurface of the electrode layer; a bonding layer that is provided on asurface of the low-strength layer; and a conductive plate that isprovided on a surface of the bonding layer, wherein strength of thebonding layer is higher than strength of the electrode layer, andstrength of the low-strength layer is lower than the strength of theelectrode layer.
 2. The semiconductor device according to claim 1,wherein the electrode layer is made of AlSi, and the low-strength layeris made of Al.